Chapter 5 - Trigonometric Identities - Section 5.6 Half-Angle Identities - 5.6 Exercises - Page 244: 77

$AB$ is a radius of the circle and $BD$ is a radius of the circle. Therefore, $AB = BD$. Since the two sides $AB$ and $BD$ are equal, the triangle $ABD$ is an isosceles triangle.

$AB$ is a radius of the circle and $BD$ is a radius of the circle. Therefore, $AB = BD$. In the triangle $ABD$, since the two sides $AB$ and $BD$ are equal, the triangle $ABD$ is an isosceles triangle.